Elf on the shelf debate with my mom

My fiancè has a 6 year old son. My mom was excited about the whole thing because she really wants to be a grandmother and I already stated I was never having biological kids. I even got my tubes tied and when I tell you how hard it was to find a doctor who would tie my tubes but my also child-free best friends husband could just get a vasectomy. That's a whole different post. Anyway he has a 6 year old son who has trauma. My mom is so excited about finally having a grandchild and really wants to do elf on the shelf. My fiancè doesn't do elf on the shelf because my step son has trauma and the idea of someone watching him like that will cause destress. I told my mom and she thinks it's harmless. She said she already bought the elf for her home. We said we aren't doing that. She thinks we are being dramatic and that kids can't live in their trauma. I agree but if it's something as simple as not doing a made up thing that isn't real then why would we do it when we know it will cause him distress? What do you guys think?